1. Field of Invention
The present invention relates to a reflective mask blank for EUV (Extreme Ultraviolet) lithography (hereinafter referred to also as “EUV mask blank” in this specification) to be used for e.g. production of semiconductors, and a process for its production, as well as an EUV mask obtained by patterning the EUV mask blank.
2. Discussion of Background
Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit with a fine pattern on e.g. a silicon substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices is being accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF laser (193 nm) is employed, about 45 nm is presumed to be the limit. Under the circumstances, as an exposure technique for the next generation employing an exposure wavelength shorter than 45 nm, EUV lithography is expected to be prospective, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser. In this specification, EUV light is meant for light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically for light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive index of substances at such a wavelength is close to 1, whereby it is not possible to use a conventional refractive optical system like photolithography employing visible light or ultraviolet light. Therefore, in EUV lithography, a reflective optical system, i.e. a reflective photomask and mirror, is employed.
A mask blank is a laminate before pattering, to be used for the production of a photomask. In the case of an EUV mask blank, it has a structure wherein a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, are formed in this order on a substrate made of e.g. glass.
Usually, a protective layer is formed between the above reflective layer and the absorber layer. Such a protective layer is provided for the purpose of protecting the reflective layer, so that the reflective layer will not be damaged by an etching process to be carried out for the purpose of forming a pattern in the absorber layer.
As the reflective layer, it is common to use a Mo/Si multilayer reflective film having a molybdenum (Mo) layer as a low refractive index layer and a silicon (Si) layer as a high refractive index layer alternately stacked to have the light reflectivity improved when the layer surface is irradiated with EUV light.
For the absorber layer, a material having a high absorption coefficient to EUV light, specifically e.g. a material containing chromium (Cr) or tantalum (Ta) as the main component, is used.
With respect to the absorber layer of an EUV mask blank, if its surface is poor in smoothness, edge roughness of a pattern formed on the absorber layer surface tends to be large, whereby the dimensional precision of the pattern tends to be low. The influence of the edge roughness tends to be distinct as the pattern becomes fine, and therefore, the absorber layer surface is required to be flat and smooth.
In order to make the absorber layer surface to be a surface excellent in smoothness, the crystal state of the absorber layer is said to be preferably an amorphous structure or a microcrystalline structure (Patent Documents 1 and 2). The present applicants have found it possible to obtain an absorber layer excellent in smoothness of the surface wherein the crystal state is an amorphous structure or a microcrystalline structure, by making the absorber layer to be a TaNH film containing tantalum (Ta), nitrogen (N) and hydrogen (H) in a specific ratio, or a TaBSiNH film containing tantalum (Ta), boron (B), silicon (Si), nitrogen (N) and hydrogen (H) in a specific ratio (Patent Documents 3 and 4).
In Patent Documents 3 and 4, between a Mo/Si multilayer reflective film and an absorber layer (a TaNH film or a TaBSiNH film), a protective layer is provided for the purpose of protecting the Mo/Si multilayer reflective film, so that at the time of forming a pattern on the absorber layer by an etching process, usually by a dry etching process, the Mo/Si multilayer reflective film will not be damaged by the etching process. As the material to constitute the protective layer, a material which is less susceptible to an influence by the etching process of the absorber layer i.e. one having an etching rate slower than the absorber layer and yet being less susceptible to a damage by the etching process, is selected for use. Further, the protective layer itself should also preferably have a high EUV light reflectivity in order not to impair the EUV light reflectivity of the Mo/Si multilayer reflective film even after the formation of the protective layer. For this purpose, In Patent Documents 3 and 4, a Ru layer or a Ru compound (RuB, RuNb or RuZr) layer is said to be preferred as the protective layer.